Single mode, fiber-optic cables have an inherent theoretical bandwidth that is more than a thousand times higher than that of electrical cables. One optical fiber could potentially carry a data stream at about ten Tera-bits per second. This is more than the total bandwidth of electronic switches used to interconnect multiple computers (hosts or "nodes") in high end servers, such as Digital's Memory Channel, IBM's SP-2 family of super-computers, and SGI's Cray T3E. Therefore, systems of "nodes" interconnected by fiber-optic cables are ideal for a very high performance system area networks (SAN), network of workstations (NOW), multi-processor communication subsystems, Internet switches, routers, and the like.
However, data are generally produced by electronic systems and need to be delivered at their destination in electronic form. Consequently, the theoretical bandwidth of a fiber-optic cable is not directly attainable because of the limitation imposed by the need to convert between signaling in the electrical domain and signaling in the optical domain. This is analogous to trying to pump water through a fire hose that is connected to straws at each end.
The general structure of an optical communication system 100 is depicted in FIG. 1. At a transmitting side, multiple, independent data streams, each of which operates at a speed attainable by electronic means, for example a source node 10, are provided on multiple input channels 101. An optical multiplexer 110 combines the multiple data streams onto a single optical transmission medium 115, generally a fiber-optic cable. At the receiving side, a demultiplexer 120 separates the combined data steam into multiple, independent data streams that are subsequently delivered to their intended destination 20 via output channels 102.
It should be noted that both the multiplexer 110 and the demultiplexer 120 may actually include a number of independent components, each responsible for only one data stream. It is further possible to combine or split the signal on the transmission medium 115 into several parts. In the case where the signals are split, each part contains all of the combined information, hence passively distributing an optical signal to multiple destinations is functionally equivalent to broadcasting.
FIG. 2 shows the schematic for the equivalent broadcasting system 200 with input channels 201, multiplexers 210, optical broadcast medium 215, demultiplexers 220, and output channels 202. The electronic nodes are not depicted.
To overcome the intrinsic limitations of the electro-optical interfaces, it is possible to multiplex multiple, independent data streams onto one fiber by using multiple different wavelengths, or by interleaving data in time. The first technique is called wavelength division multiplexing (WDM), and the second technique is called optical time domain multiplexing (OTDM). There are also other forms of optical multiplexing techniques under investigation, for example, ones based on spread-spectrum techniques, however, WDM and OTDM are the only techniques that have currently been reduced to practical applications.
Wavelength division multiplexing is similar to a conventional broadcast system. There, the electromagnetic spectrum is divided into multiple channels with a different frequency assigned to each channel. The transmitters and receivers can selectively operate on specific channels while sharing the same transmission medium without interference. Fiber-optic transmission systems using WDM have been demonstrated with up to 128 channels.
However, WDM systems are rather costly because they require highly frequency-stable lasers on the transmitting side, and very selective filters on the receiving side. Moreover, at this point in time, WDM systems either cannot change channels at all, or require a relatively long time to tune the transmitters and receivers to different frequencies, as long as many milli-seconds.
As an advantage, WDM system can operate over very long distances, and are ideal for wide area networks (WANs), telephony, cable TV systems, etc. WDM systems are not compelling at the system area network level due to their high cost, a limited number of channels, and their limited ability to switch channels. Because WDM systems cannot easily reallocate bandwidth to meet rapidly changing demands, they are best suited in situations with fairly static communication patterns.
FIG. 3 is a timing diagram of an optical domain multiplexed broadcast signal 300 for an example eight node network. A system that can produce the signal 300 is described below with reference to FIG. 4. In FIG. 3, time is indicated along the x-axis 301, and the relative intensity (amplitude) of the signal 300 is indicated along the y-axis 302. In the broadcast signal 300, pulses 310, 320, 330, 340, and so forth, are called framing pulses, and pulses 311-318 are data pulses, i.e., there are eight data pulses for each framing pulse.
Logical ones and zeros are respectively indicated by solid and open data pulses, i.e., the presence or absence of light pulses. The framing pulses are always ones. Generally, the framing pulses have a greater intensity than the data pulses so that they can readily be discerned. The rate of the framing pulses determines the bit rate of the network. The relative offset of the data pulses with respect to the framing pulses determines the OTDM channels to which transmitters and receivers can tune.
As shown in FIG. 4, the data from many sources are sequentially interleaved on a single medium, for example, an optical fiber. Because the system operates with a fixed bit rate (frequency), the time between two consecutive bits from the same source is a constant: the bit-time. The bit-time is the inverse of the transmission rate of each channel.
The data from all channels are transmitted sequentially, starting with the first bit of channel "0" (311), the first bit of channel "1" (312), etc. After the first bit from all channels have been sent, the cycle repeats with the next bit, and so on. The total number of channels in the system is a constant. The bit-time multiplied by the number of channels determines the total bandwidth of the system (total bit rate (TBR). The framing pulses delineate the channel multiplexing sequence. For example, each framing pulse may precede the data from channel "0." Each receiving node in the system can use the framing pulses to synchronize its selection mechanism based on a relative time offset from the framing pulses.
Because of the speeds involved, multiplexing and demultiplexing must be performed optically. Each transmitting node must insert a data bit in its assigned channel time slot, similarly, each receiver extracts its data from one of the channel time slots. As an advantage, an OTDM system can utilize many channels, and it is possible to change channels quickly, for example, in a few nano-seconds as opposed to many milli-seconds for an WDM system. In other words, changing channels in an OTDM system can be about a million times faster than changing channels in a WDM system). As is the case for a WDM system, the multiplexing and demultiplexing functions can be distributed such that each device that is interconnected has one multiplexing and one demultiplexing device as part of its network interface. Such a configuration is shown in FIG. 4.
FIG. 4 shows the basic structure of an optically coupled network 400 that can produce the multiplexed signals as shown in FIG. 3. This system is disclosed in U.S. Pat. No. 5,493,433 issued to Prucnal et al. on Feb. 20, 1996. Framing pulses as shown in FIG. 3 are generated by a single modelocked, pulse compressed laser source 413 of a "hub" 410. The pulse rate is equal to the bit rate that is used by each of the attached nodes, for example, 1.3 Gbits/sec. The pulse width of each light burst is considerably smaller, e.g., about one pico-second. The laser 413 acts as a central timing clock with sparse framing pulses. As described above, the empty spaces between the timing pulsing are filled with the data of the various broadcast channels synchronized to the framing pulses.
The centralized hub 410 has an input side 411 connected to an output sides 412 by an optical medium 414. The hub 410 connects the various nodes in the network. The hub 410 is configured as a passive power splitter that distributes the light pulses through it equally to all attached nodes. It should be noted that using a single laser source makes the amount of signal received by any one node inversely proportional to the number of connected nodes. In a network of 50 nodes, each node receives about 1/50 of the crucial framing pulses to synchronize data pulses on the various channels.
Each node has a receive section 401 connected to the output side 412 of the hub 410, and a transmit section 402 connected to the input side 411 of the hub 410. The input side 411 of the hub 410 can be connected to any output side connection via the medium 414 to complete any arbitrary loop between any node transmitter 402 and any node receiver 401, including itself. In other words, the system can act as a cross-bar switch or a broadcast system.
In the receive section 401, the optical signal is passed through an optical signal separator 420. The separator 420 separates the framing pulses from the data pulses, for example, according to their polarization. The framing pulses are passed through on lines 424 and 434, and the data pulses are fed to a first optical delay element 421. A particular receive channel (RxChannel#) is selected by a delay signal on line 422.
The output of the delay element 421, as well as the framing pulses on line 424, are fed to an optical AND gate 425. When the selected channel's data pulses are time aligned with the framing pulses on line 424, signals are produced on the line 428. These signals are fed to a photo-detector (DET) 426 to produce received data (RxData) as an electronic bit stream on line 427.
The transmit section 402 operates as follows. A bit stream (TxData "1011011") is supplied on line 435. The bit stream is converted to optical signals by a modulator 436 which also receives the framing pulses on line 434. A particular transmit channel (TxChannel#) is selected by line 432 connected to a second delay element 431.
The delay element 431 inserts the data signals at the appropriate time displacement between the framing pulses. From the delay element 431, the optical signal is presented to the input side of the central hub 413 where the light bursts are combined with those transmitted by other nodes. The combined broadcast signal 300 as shown in FIG. 3 is subsequently distributed to all nodes.
As a feature of this transceiver arrangement, all devices that are operated by electronic signals only need to operate at the channel bit rate. On the optical broadcast medium itself, the data bits are fitted in the time multiplexed channels between the framing pulses as shown in FIG. 3. In addition, the synchronicity inherent in an OTDM protocol allows for efficient control and arbitration operations. Every operation can be tuned to the same timing signal.
However, there are some problems with this arrangement. Because only a single centralized laser is used for all framing pulses, the arrangement requires a large dynamic range and good extinction ratio of the optical AND gate, both of which are difficult to achieve. This limits the total number of nodes that can concurrently transmit and receive. It is relatively difficult to extract the framing pulses from the input data stream, particularly when the exact required orientation of the planes of polarization of the framing and data pulses is lost due to uncontrollable bifringence in the fiber. In order to increase the number of nodes in the network, a considerable more expensive laser needs to be used, regardless of how many nodes are actually present.
The pulses must be distributed to all nodes such that the delay between the centralized laser pulse source and each node is controlled to within a small fraction of the channel width. This is difficult because the channel width is only a few pico-seconds. This time corresponds to a distance of much less than less than one millimeter for a pulse traveling at the speed of light. Given the physical size of the system, stress on the fibers, temperature variations and other environmental factors, it is required to actively control the delay from this central clock source to each node in the system.
To insure that the signal from the central clock source, i.e., the framing pulses, experience the same delays as the data, it ought to use the same optical path. However, this approach requires that this pulse must be separated from the data at the receiving node by some means. This leads to the problem of separating the data and the clock pulses by optical means, which is quite difficult and which may leave residual signals, which in turn reduce the signal-to-noise ratio of the entire system. Proposals that include orthogonal polarizations are prohibitively expensive. Proposals that try to distinguish between framing and data pulses by virtue of different amplitudes face dynamic range problems. Proposals that employ different wavelengths experience differences in the signal propagation speeds due to fiber dispersions in addition to facing all the problems of WDM systems and the needs for complex wavelength converters.
The power level of a central optical clock source shared by all nodes in the system must be designed to meet the requirements of the maximal system configuration size and represents a significant up-front investment cost, even when only a small number of nodes are present. The power levels available at each node in a system with one central optical clock source is relatively small and limits the choice of optical components used to isolate one channel from the combined OTDM traffic.
Previously described systems use polarization maintaining (PM) fibers, which are rather costly. In addition, the propagation speed of PM fibers differs slightly for the two polarizations, which requires compensations that is dependent on the distance between each node and the central clock source.
Therefore, there is a need for an optically coupled system that does not depend on a single centralized laser source at a hub.